The demand for pure hydrogen is rapidly increasing in, for example, the electronics industry and further major growth is anticipated in the emerging market of low temperature fuel cells. Methanol, though more costly than natural gas, has a unique combination of characteristics as "hydrogen source", including easy transportation, handling, safe and low cost storage. Methanol is thus particularly suited for on-site pure hydrogen generation by catalytic steam reforming. The well known catalytic carbon monoxide shift reaction is, presumably, a step in methanol-like fuel reforming, as later described. In general, carbon monoxide, with or without admixed hydrogen, such as is produced by partial oxidation of carbon (e.g. coal) or of methane, respectively, is a similar hydrogen source; carbon monoxide, as well as methanol itself, being subject to substantially higher conversions at much lower steam reforming temperatures than natural gas and other hydrocarbons; and they lend themselves to simpler energy efficiencies.
The prior art is replete with descriptions of steam reforming of methanol on a copper-bearing catalyst at ca. 150.degree. C.-350.degree. C., as illustrated, for example, in three publications by J. C. Amphlett and associates, entitled "Hydrogen Production by the Catalytic Steam Reforming of Methanol Parts 1, 2 & 3" [The Canadian Journal of Chemical Engineering, 59, 720-727 (1981); 63,, 605-611 (1985); and 66, 950-956 (1988) respectively], and in two publications by R. O. Idem and N. N. Bakhshi, entitled "Production of Hydrogen from Methanol. 1. & 2." [Ind. Eng. Chem. Res. 33, 2047-2055 and 2056-2065, (1994)], incorporated herein by reference.
Generally, it is understood that methanol is first decomposed to carbon monoxide and hydrogen according to EQU CH.sub.3 OH=CO+2H.sub.2 (1)
followed by the also well-known carbon monoxide "shift" reaction with steam, EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2 (2)
resulting in the overall equation EQU CH.sub.3 OH+H.sub.2 O=CO.sub.2 +3H.sub.2 (3)
Unavoidably, some carbon monoxide remains unreacted. The temperature is maintained between about 150.degree. C. and 350.degree. C. to attain reasonable kinetics and catalyst endurance. Within this range, methanol conversion increases with temperature, but so also does carbon monoxide formation, up to as high as a few percent. As shown, for example, in the R. O. Idem et al publication (page 2061), with the stoichiometric 1:1 methanol/water feed, the carbon monoxide content is between 1.2 and 1.9% (12,000 to 19,000 ppm) at 250.degree. C. with about 74% methanol conversion depending on the particular copper catalyst. Excess steam over the stoichiometric ratio of 1:1 increases the conversion and reduces the carbon monoxide content, but adds to the vaporization heat input. The excess steam effect is illustrated in Table 1 of an article by O. A. Belsey, C. M. Seymour, R. A. J. Dams and S. C. Moore [Electrochemical Engineering and the Environment 92, Hemisphere Publishing Corporation, page 52, 1992] which shows, with a water-to-methanol ratio of about 3.6, a conversion of 94.3% and 800 ppm of carbon monoxide at the low temperatures of 230.degree. C. at the catalyst wall and 183-195.degree. C. across the catalyst bed, and of 99.36% conversion and 4000 ppm of carbon monoxide at the high temperature of 300.degree. C. at the wall and 195-235.degree. C. across the bed. Similarly, copper-based low-temperature shift catalysts, such as described in "Heterogeneous Catalysis in Practice" by C. N. Satterfield, McGraw-Hill, Inc. pages 294-295 (1980), result in similar levels of carbon monoxide. To avoid excessive sintering, the copper-based catalysts should be operated below about 325.degree. C., and preferably below about 300.degree. C.
The prior art is also replete with descriptions of catalytic reactions coupled with selective hydrogen permeation across, among others, palladium alloy membranes. The following discussed citations are believed to be representative of the state-of-the-art in this area.
Reference is made, for example, to "Catalysis with Permselective Inorganic Membranes", by J. N. Armor, a 25-page Review published by Elsevier Science Publishers B.V. in 1989; to an article entitled "Catalytic Palladium-based Membrane Reactors: A Review," J. Shu, B. P. A. Grandjean, A. Van Neste and S. Kaliaguine, The Canadian Journal of Chemical Engineering, 69, 1036-1059 (1991); to an article entitled "Catalytic Inorganic-Membrane Reactors: Present Experience and Future Opportunities", by G. Saracco and V. Specchia, CATAL. REV. - SCI. ENG., 36 (2), 305-384 (1994); and to yet another review entitled "Current hurdles to the success of high-temperature membrane reactors", by G. Saracco, G. F. Versteeg and W. P. M. Swaaj, J. of Membrane Science, 95, 105-123 (1994), all incorporated herein by reference. The emphasis in the above cited art has been on enhancing hydrogenation and dehydrogenation reactions.
Reference is also made to U.S. Pat. No. 4,810,485 to Marianowski et al., which describes broadly in situ hydrogen production and membrane purification, naming hydrocarbon steam reforming reactions "such as reforming of methane, propane, ethane, methanol, natural gas and refinery gas; water-gas shift reactions; and carbonaceous material gasification reactions, such as gasification of coal, peat and shale" (Col. 2, 1.60-65); and metallic membrane foils, stating that "suitable metals include palladium, nickel, cobalt, iron, ruthenium, rhodium, osmium, iridium, platinum titanium, zirconium, hafnium, vanadium, niobium, tantalum, copper , silver and gold and alloys thereof, particularly palladium, copper nickel and palladium silver alloys." (Col. 3,1.57-62). The patentees further state that "foils of copper, nickel and mixtures thereof are particularly preferred - - - ", and speculations are offered limited to high temperatures above 1000.degree. F. or 538.degree. C. and to membrane thicknesses of 0.0001 and 0.001 inch. There is no teaching in this patent or even a hint of the ruinous flux deterioration and the potentially catastrophic carbon monoxide poisoning described more fully hereinafter, which occur at the much lower temperatures with which the present invention is concerned and which have, surprisingly, been solved thereby. Moreover, with the exception of palladium-bearing membranes, the patentees membranes made of other metals are useless for the purposes of the present invention, including their preferred copper and nickel membranes, which are impermeable to hydrogen at the relatively low temperatures of the present invention.
As later more fully explained, the present invention is concerned with linking steam reforming of a methanol-like fuel to pure hydrogen generation by permeation through selective membranes, a concept that differs radically from mere prior art coupling hydrogenation and/or dehydrogenation with such membranes. By way of explanation, as illustrated generically in FIG. 11, page 1051 of the J. Shu et al publication, either pressurized hydrogen is permeated from the "upstream" across the membrane into the "downstream", where it hydrogenates the reactant in the presence or absence of a catalyst; or, in dehydrogenation, hydrogen is extracted upstream from the reactant, also in the presence or absence of a catalyst, and permeated to the downstream side, where it becomes merely an impurity in a sweep gas.
In neither case, however, is pure hydrogen generated downstream, at a usable (i.e. at least atmospheric) pressure, by permeation from its mixture with the much heavier carbon oxides and steam upstream; the latter interfering seriously with hydrogen permeation rates at the moderate methanol reforming temperatures, as more fully later discussed.
The J. Shu et al. article discloses also methanol reforming in conjunction with palladium alloys (Table 5, page 1046), referring to J. E. Philpott's article "Hydrogen Diffusion Technology" [Platinum Metals Review, 29, (1), 12-16 (1985)]. Philpott outlines here the commercial application of "the technology for extracting pure hydrogen from hydrogen-rich gas mixtures by diffusion - - - " through the walls of closed-end palladium/silver tubes, including the catalytic methanol/steam reaction.
Over a decade earlier, A. G. Dixon, A. C. Houston and J. K. Johnson have described "an automatic generator for the production of pure hydrogen from methanol" [Intersoc. Energy Convers. Eng. Conf., Conf. Proc. 7th, 10084-1090 (1972)]. Specifically, 1:1 molar methanol/water is fed under pressure to a catalytic reactor where the reformate is produced at a temperature between 275.degree. C. and 325.degree. C. In this temperature range and at moderate pressures, methanol conversion is substantial, resulting in a mixture of hydrogen and carbon dioxide close to the stoichiometric ratio of 3:1, but containing some carbon monoxide as well as some residual reactants. The reformate is then fed to a tubular palladium/silver diffusion cell wherein pure hydrogen is permeated under a hydrogen pressure gradient through its wall which is the hydrogen selective membrane. The diffusion cell is suspended integrally with the reactor "in an externally heated fluidized bed with sand as a heat transfer medium" (1st para., lines.6-8), so that reforming and permeation occur at a single temperature.
Earlier this year we have received and undated brochure by Johnson Matthey (Singapore) Pte Ltd entitled "Hydrogen Generation Systems". The brochure mentions units operated for more than 15 years and depicts the flow sheet of their commercial catalytic hydrogen generator according to which a 1:1 molar methanol/water mixture is preheated in a heat exchanger (cooling the hot pure hydrogen product), pressurized and vaporized in a heated reactor and reformed therein over a catalyst to a mixture of, mainly, hydrogen and carbon dioxide, and also containing several thousand parts per million (herein "ppm") of carbon monoxide; the reformate being then fed to a tubular diffuser containing an inner concentric also closed-end palladium/silver tube into which pure hydrogen product is permeated from the reformate with the waste gases vented.
With respect to the shift reaction (equation (2), above), reference is made to a paper entitled "The Water Gas Reaction Assisted by a Palladium Membrane Reactor" by S. Uemiya, N. Sato, H. Ando and E. Kikehi [Ind. Eng. Chem. Res., 30, 589-591 (1991)], which describes the higher carbon monoxide conversion attained by permeating the product hydrogen across the membrane and removing it downstream by means of an argon sweep gas. The membrane here was "a composite structure consisting of thin palladium film (palladium thickness 20 microns) supported on the outer surface of a porous-glass cylinder", the preparation of which was described in an earlier paper [Chem. Lett.,489-492, (1988)] by Uemiya et al.
Among typically thin palladium-bearing membranes of the prior art, reference is made to the early publication by R. Goto entitled "Hyperpure Hydrogen from Palladium Alloy Permeation (1)", Chemical Economy & Engineering Review, 2 (10) 44-50, 1970, which compares, inter alia, the "flux", "Q", i.e. the rate of hydrogen permeation from pure hydrogen upstream under pressure, through several palladium alloys (Table 1, page 46). Certain thin palladium/silver and palladium/copper alloys are disclosed, for example, in the more recent Shu et al. publication, (pages 1041-1042) and also palladium/ruthenium alloys in the Armor publication, (page 17), such being also incorporated herein by reference. A comprehensive up-to-date review of hydrogen selective palladium bearing-membranes, among others, appears in U.S. Pat. No. 5,498,278 (Mar. 12, 1996), of which, specifically, the palladium bearing metal membranes made by "coating certain less expensive transition metal alloy base metals with palladium or palladium alloys" (Col.3, lines 22-24), are also incorporated herein by reference.
The methanol-based pure hydrogen generation of the prior art, utilizing palladium bearing-membranes, however, has serious technical shortcomings, one of which is caused by the significant amounts of carbon dioxide and steam in the reformate. Specifically, we have found that hydrogen fluxes are much lower in their presence than in their absence, within the temperature range of 150.degree. C. to 350.degree. C.; and that the inevitable presence of carbon monoxide causes a further flux loss by poisoning palladium-bearing membranes at moderate temperatures, as carbon monoxide is preferentially adsorbed over hydrogen. This loss increases drastically with decreasing temperatures between about 300.degree. C. to 150.degree. C., as well as with increasing carbon monoxide concentration, resulting in carbon formation and eventually in membrane failure and/or total flux loss. In the before-described shift reaction, the much higher concentration of carbon dioxide admixed with steam causes an even more drastic aggravation of membrane poisoning.
The term "pure hydrogen flux", as used herein, is its rate of permeation in terms of volume of hydrogen per unit of membrane area per unit of time, e.g. cubic centimeters per square centimeter per minute (cc/cm.sup.2 -min).
The carbon monoxide poisoning effect, as a function of temperature and carbon monoxide concentration, is illustrated in Tables 1 & 2; and the aggravation due to the presence of carbon dioxide and steam is shown in Table 3.
In the Tables, there are shown the ratios of the flux, Q, of the hydrogen permeating from the reformate to the flux, Q.degree., permeating from pure hydrogen. The higher Q/Q.degree., the better the permeation, up to, at the limit, equal fluxes, or Q/Q.degree.=1.
TABLE 1 Poisoning of a Pd/25% Ag membrane by a mixture of 98.4% hydrogen and 1.6% CO after 100 hours of operation vs. temperature. Temperature (.degree. C.) Q/Q.degree. 330 0.98 300 0.85 275 0.65 250 0.17 200 0.03 150 zero
TABLE 2 Poisoning (same membrane) by 1%, 1.6%, and 2% CO in hydrogen after one and one-half hour operation at 150.degree. C. CO concentration Q/Q.degree. 1.0% 0.32 1.6% 0.18 2.0% 0.02
TABLE 3 Aggravation of CO-poisoning by carbon dioxide and steam, after two and one half-hours of operation at 200.degree. C. gas composition (ex H.sub.2) Q/Q.degree. 2% CO 0.4 1.6% CO + 12% H.sub.2 O 0.01 1.6% CO + 6% CO.sub.2 0.27 1.6% CO + 22% CO.sub.2 0.07
Evidently, with the palladium/silver alloy, the operating temperature must be maintained at least at about 275.degree. C. and preferably above 300.degree. C., to minimize or prevent carbon monoxide poisoning of the membrane.
Other alloys, while generally subject to such poisoning, differ in their resistance thereto as illustrated below. A substantially non-poisoning temperature, as defined herein, is one at which the flux loss is tolerably slow with time, without carbon formation, thus being "reversible", i.e. for example by occasional oxidation of adsorbed carbon monoxide or by temporary exposure to pure hydrogen.
In the prior art, furthermore, the hydrogen in a methanol reformate is permeated from an outer otherwise empty tube into an inner concentric, e.g. palladium/silver alloy membrane tube at a flux so inadequate as to make the process uneconomical for widespread use, and so also is the flux of the hydrogen permeated from the product of the shift reaction, herein also sometimes referred to as "shift reformate".